The new importlib shows a regression w.r.t. previous CPython versions. It no longer recognises an "__init__.so" file as a package. All previous CPython versions have always tested first for an extension file before testing for a .py/.pyc file. The new importlib explicitly excludes the ExtensionFileLoader from loading packages. See importlib/_bootstrap.py, line 1579 onwards:

   1579 def _get_supported_file_loaders():
   1580     """Returns a list of file-based module loaders.
   1581 
   1582     Each item is a tuple (loader, suffixes, allow_packages).
   1583     """
   1584     extensions = ExtensionFileLoader, _imp.extension_suffixes(), False    # <== bug here
   1585     source = SourceFileLoader, SOURCE_SUFFIXES, True
   1586     bytecode = SourcelessFileLoader, BYTECODE_SUFFIXES, True
   1587     return [extensions, source, bytecode]

(BTW, I'm not sure what to file this bug against - "Interpreter Core" or "Library"?)